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THE SEA VERSES YOUR BOAT

1. Osmotic Blistering. Osmosis is the movement of water molecules across a partially-permeable membrane due to a water potential gradient. More specifically, it is the movement of water across the membrane from an area of high solute concentration to an area of low solute concentration. In the case of yachting, the high solute concentration refers to chemical concentrations in trace moisture in the lay-up, and sea or fresh water makes up the low solute concentration. The common polyester gel-coat hull is not, in fact, entirely waterproof, and it acts as the semi-permeable membrane (epoxy hulls, or epoxy coated hulls, were the solution).

2. Galvanic corrosion. All ferrite metals will oxidize very obviously around sea water, however two different metals in contact with each other and immersed in sea water will form an electrical closed circuit, with current (and metal) going from the less noble metal to the more noble metal. This is called galvanic corrosion. It is applied, in industry, for electroplating. Dissimilar metals will also react only in close proximity to each other, while being submerged in seawater. The 'Table of Nobility', shown below, indicates relative similarity between metals, as well as their potential. Note how far down the list Zinc is from most common yachting metals, which explains why it is used as sacrificial anodes in marine applications.

Suffice to say, the closer the two metals are in positioning on this list, the less they will react to each other. Of particular interest is, obviously, stainless steels and their combinations with other common marine metals such as bronze, aluminum, and brass. In these cases, only bronze is a safe match for stainless, with aluminum often becoming pitted and brass (in one of the worst mistakes made by yacht owners) often getting confused with bronze with disastrous results. Made of copper and zinc, brass will loose the zinc first, becoming brittle and fracturing before the owner notices anything wrong. It is common on through hull shut off valves.

NEGATIVE END (-) MORE NOBLE
Platinum
Gold
Graphite - note1
Silver
Titanium
Hastelloy C
Stainless steel type 312 (passive)
Stainless steel 317 (passive)
Stainless steel 316 (passive)
Stainless steel 304 (passive)
Stainless steel 302 (passive)
Nichrome
Inconel
Nickel
Silver Solder
Monel K-500
Monel 400
Cupronickel 70-30
Cupronickel 90-10
Nickel aluminum bronze
Silicon bronze
Copper
Aluminum bronze
Red brass
Admiralty brass
Yellow brass
Inconel (active)
Nickel (active)
Naval brass (aka Tobin bronze)
Manganese bronze - note 2
Muntz metal
Tin
Lead
Stainless steel 316 (active)
50-50 lead-tin solder
Cast iron
Wrought iron
Mild steel
Cadmium
Aluminum alloys
Aluminum (pure)
Galvanized steel
Zinc
Magnesium
POSITIVE END (-) LESS NOBLE

3. Ultraviolet Degradation. UV degradation is a form of polymer degradation which affects plastics used in yachting. Sensitive polymers include polystyrene, polyethylene, nylon, synthetic rubber, silicone, etc., as well as specialty fibers like aramids. Although considered exotic fibers, aramids are heavy used in yachting, represented by Kevlar, Dyneema and Spectra sailcloth and laminates, as well as running and sometimes standing rigging. UV absorption leads to chain degradation and loss of strength at sensitive points in the chain structure of polymers. Pigment degradation occurs in conjunction, but often a weakened plastic will show no visible signs before failure